I found a strange thing.
1. I change Black Grail to another unit in last but one mission (B4_10)
2. I win the battle, unit with Id = "Id of Black Grail" 257 is actually destroyed fully.
3. In after-battle screen I get message: Victory, Undead Forces Destroyed, Black Grail destroyed.
4. However if I go to last mission, that victory in b4_10 does not count, and Black Grail is present in last battle.
If I did not change Black Grail to another unit, then everything foes fine: Black Grail is counted as defeated and is not present in last mission.
Any idea what else (except unit id=257) is checked by Dark Omen? Maybe portrait of a Black Grail Knight? or what?.